The invention is directed to cobalt alloys for the production of fixed and removable dental constructions consisting of (or consisting essentially of) 42 to 69.5 weight % cobalt, 10 to 35 weight % chromium, and 5 to 25 weight % tungsten.
For a long time there have been approved for the production of crowns, bridges, and inlays for restorative dentistry noble metal alloys based on gold, platinum, palladium, and silver. This is especially true for the so-called porcelain fused to metal alloys in which for esthetic reasons, the metallic crowns and bridges are entirely or partially veneered with tooth colored ceramics.
However, for reasons of cost for many years they have given way to an increasing extent to non-noble metal alloys. The great majority of these alloys are so-called Ni based alloys which in addition to nickel, contain as an essential additive chromium for the production of a sufficient resistance to corrosion and oxidation. The composition of such alloys is known, for example, from European Pat. No. 5013, German OS 2432014, German OS 2538099, German OS 2528547, and U.S. Pat. No. 3,948,653.
Recently, however, doubts have been exposed in increasing measure to the use of nickel based alloys which doubts are caused by the known allergic action of nickel or the known or believed carcinogenic action of nickel dusts or nickel compounds (see MAK-Werte-Liste der Berufsgenossenschaft der Chemischen Industrie 1979 and S. S. Brown, F. W. Sundermann; "Nickel Toxicoloty," Academic Press, London 1980).
Cobalt based alloys have proven good in dental medicine for many years, for example, for the production of cast palate plates for removable dental structures. Allergic reactions have not been known by patients who wear this type of prothesis. Therefore there have not been a lack of attempts to also produce crowns and bridges for the complete or partial veneering made of cobalt alloys with ceramics.
A series of requirements are placed on the so-called fired on alloys.
Paramount among these is a thermal expansion and contraction behavior which is balanced to the type of dental ceramic used, which customarily is fused on the metallic framework in several layers at about 950.degree. C. to 1000.degree. C. A second essential requirement of dental porcelain fused to metal alloys is that the alloys are not too hard. Alloys having hardnesses of about 300 (HV 5) lead to considerable difficulties in separating, grinding, milling, and polishing in dental laboratories and therefore to increased wear of the work tools and longer time requirements which can partially or completely eliminate the price advantages compared to noble metal alloys. Furthermore, alloys having hardnesses of more than about 300 (HV 5) can no longer be regarded as physiologically compatible. Frequently; in the side tooth area crowns and bridges are not completely veneered In order to protect the antagonists before there is too high an abrasion, the masticating surfaces should not be veneered with the very hard ceramics. This mode of action from experience is only meaningful if the hardness of the unveneered metallic masticating surfaces are clearly below 300 (HV 5).
Further important requirements of alloys fused to dental porcelain are that the alloys be meltable and castable easily and without problem in the casting apparatus customary in the dental laboratory. This means that dental alloys must be meltable and castable, e.g. with gas burners, inductive high frequency casting apparatus, electric arc melting apparatuses and suitable electrically heated casting devices. Therefore alloys which because of their compositions, can only be melted and cooled under a vacuum or protective gas, e.g. perhaps only in very specific and expensive crucible materials are not practical. A good form filling and flow behavior of the alloys in connection with the castability still has a special role because crowns and bridges in part must have a very filigree form. Additional decisive requirements are good solderability and a good exact fitting of the cast objects.
A good adhesion to the ceramic which must guarantee an internal union between the two material components which withstands all loads also is of great significance for dental porcelain fused to alloys.
All previously known cobalt alloys contain in addition to cobalt as the main component chromium as well as secondary components, molybdenum, and/or tungsten as well as additional elements. Examples of tungsten containing alloys are found in U.S. Pat. Nos. 4,229,215; 4,263,045; 4,255,190,; and 4,253,869, and in German OS 3001126 and 3038036.
In several of the mentioned alloys there are specified as essential components noble metals such as ruthenium.
However, all known dental porcelain fused to alloys based on cobalt have in part considerable inadequacies in one or more of the essential properties mentioned for dental porcelain fused to alloys. This is especially true in regard to their compatability with known types of veneering ceramics and their hardness. Thus all known fused to procelain alloys based on cobalt show hardnesses of at least about 300 (HV 5) and in some cases, of over 450 (HV 5). Many of the known cobalt fused to porcelain alloys when used with specific types of ceramics moreover, especially with rapid cooling from firing temperature (about 960.degree. C. to 1000.degree. C.) lead to cracks in the ceramic. With many known cobalt alloys fused to porcelain, moreover, the adhesion between alloy and ceramic is not sufficient so that frequently it is necessary to use adhesive materials.
Therefore, it was the problem of the present invention to develop cobalt alloys for the production of fused and removable dental constructions containing (or consisting essentially of) 42 to 69.5 weight % cobalt, 10 to 35 weight % chromium, and 5 to 25 weight % tungsten which have hardnesses of clearly less than 300 (H V5) after the heat treatment necessary for the veneering with ceramic, which are fused to the known types of ceramics in crack-free and well adhering manner and which can be worked in problem-free and simple manner with the customary methods of working and apparatuses in the dental laboratory.